Nudibranchs inside egg cases

July 17, 2000
From: Denise Nielsen Tackett

I am a professional underwater photographer with a keen interest in marine life. Over the past five years I have dived mainly in Lembeh Strait in North Sulawesi, Indonesia and have seen and photographed something that interests and puzzles me. I am attaching 3 photos of egg cases (I think they are from worms or slugs) that have tiny nudibranchs inside them. The egg cases occur on sandy bottoms in very shallow water (3 - 20 meters deep). I have observed this over several years, mostly from July to December, but at other times as well. The egg cases seem to belong to different species because the egg pattern and sac shape differs slightly. Each sac seems to have several nudibranchs in it. The nudis are aeolids. Are they feeding on the eggs? Are there any nudis that do not have a veliger stage and hatch from egg cases like these?

Re the photos: UPPER RIGHT. Shows the entire sac/case with many nudis inside (size range 1-5mm long). The nudis are actually inside the sac that is why they are not super sharp - I'm shooting through the egg case. Can you shed any light on this for me. LOWER PHOTOS: Upper left - animals in egg case. Lower left - dorsal view of animal (approx 5mm long) on outside of egg case. Right - ventral view of animal (approx 2mm long) crawling on inside of egg case. White spots are polychaete eggs.

Many thanks,

Nielsen Tackett, D., 2000 (Jul 17) Nudibranchs inside egg cases. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

Dear Denise,
I have been waiting 30 years for some more of these animals to turn up. A fish biologist found these aeolids in a similar egg mass on the Great Barrier Reef in 1976. He also photographed their egg mass, attached to the inside of the one they were eating, and collected a carnivorous nudibranch of the genus Gymnodoris which was probably eating the aeolids. If he gives me permission, I will post his photos on the Forum.

Your animal is a species of Favorinus, a genus of aeolids which feed almost exclusively on eggs, usually of other sea slugs, but sometimes of snails. Have a look at the page on Favorinus japonicus for a typical species. In this case, their food is the eggs of a polychaete worm. The eggs are much smaller than those of an opisthobranch, and they do not seem to be in individual capsules.

One family of opisthobranchs which have a similar egg mass is the Aglajidae, which includes the tropical genera Chelidonura and Philinopsis, but in aglajids the eggs are in capsules and the capsules are linked together by a short string or chalaza. The eggs are arranged in a spiral within the egg sac. Have a look at Mary Jane Adam's photo of the egg masses of Chelidonura inornata.

I look forward to seeing some specimens,
Best wishes,
Bill Rudman.

Rudman, W.B., 2000 (Jul 17). Comment on Nudibranchs inside egg cases by Denise Nielsen Tackett. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from


Favorinus sp. 3

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